1. Field of the Invention
The present invention relates to a piezoelectric vibrator having a bevel or convex configuration and using thickness-mode vibration as the primary vibration.
2. Description of the Prior Art
Piezoelectric vibrators have been in use as the sources of clock in various types of electronic equipment, many of them in small size, or rectangular types especially in the case of piezoelectric vibrators for surface mounting. For vibrators to be used in the sources of clock, a stable temperature characteristic and a high mechanical quality factor Q are required to avoid an abrupt change of the vibration frequency due to the coupling at a certain temperature of the primary vibration (typically thickness mode such as thickness-sliding vibration and thickness longitudinal vibration) and spurious vibration.
With piezoelectric vibrators in which the primary vibration is thickness-sliding vibration such as using AT-cut crystal plate or X-plate of lithium tantalate or in which the primary vibration is thickness longitudinal vibration such as using Z-plate of lithium niobate, for example, the spurious vibration can be suppressed by locally forming a vibration-excitation electrode on the primary plane of the vibrator. This makes use of the phenomenon of vibration energy being confined only underneath the vibration-excitation electrode when a specific vibration mode of a specific piezoelectric material is used as the primary vibration by locally forming the vibration-excitation electrode on the primary plane of the vibrator. Such a vibrator is called an energy-confining type vibrator.
This phenomenon is described in the following by referring to FIG. 3 which is a cross-sectional view of a piezoelectric vibrator. Assuming in FIG. 3 that the cut-off frequency of the portion where a vibration-excitation electrode 6 is formed is f0 and that of the non-electrode portion is f0', at a frequency higher than f0' the vibration energy is freely propagated without making a standing wave even underneath the vibration-excitation electrode. However, in the range of frequency higher than f0 and lower than f0', though vibration energy freely propagates on the portion the vibration-excitation electrode 6 is formed, it exponentially attenuates on the non-electrode portion and the vibration displacement declines as one goes closer to the end portions of the vibrator. Unless the amount of attenuation of the vibration energy on the non-electrode portion is large enough, or unless the magnitude of vibration displacement at the end portions of the vibrator is small enough, spurious vibration is produced by the waves reflected at the end portions of the vibrator thus deteriorating the characteristic.
Consequently, in consideration of the requirement to provide a non-electrode portion having certain minimum dimensions in order to suppress such spurious vibration and to obtain a high mechanical quality factor Q and the fact that the dimensions of the vibration-excitation electrode are determined by several conditions including securing of a low resonant impedance, it is self-evident that a certain minimum vibrator length is required. For example, in the case of a thickness-sliding vibrator using an X-plate of lithium tantalate, the ratio L/H of the longitudinal length L of the vibrator to the thickness H of the vibrator is required to be 14 or greater (ref. Japanese Laid-Open Patent Application No. Sho 58-190115).
Among the methods to increase the degree of attenuation of the vibration displacement at the end portions of a vibrator, a method to form the end portions of the vibrator to a bevel or convex configuration is well known. However, by making the end portions of the vibrator to a bevel or convex configuration, the thicknesses of the end portions of the vibrator become small causing a problem in the mechanical strength and leading to chipping or cracking. Furthermore, when it comes to mounting on a board by the conventional method of securing a piezoelectric vibrator by electrically conductive adhesive and the like, it becomes harder to steadily mount a piezoelectric vibrator on a board because of the added problem of dispersion of electrical characteristic such as the resonant frequency and resonant impedance due to dispersion of coating condition of the electrically conducting adhesive and the like especially for smaller-size piezoelectric vibrators .
As a result, as a method to address these problems, a piezoelectric vibrator is proposed in which a vibrator section and a support section are integrated. For instance, in a thickness-sliding vibrator using an AT-cut crystal plate, a slit is provided by sand blasting process on the end portions in the direction perpendicular to the longitudinal direction of the vibrator which is the direction of propagation of the primary vibration, or the direction of width of the vibrator, to allow integration of only the longitudinal end portions with the support section (ref. Japanese Laid-Open Patent Application No. Hei 5-259799).
As it was necessary to make the length of the non-electrode portion large in order to sufficiently confine vibration energy and to make the vibration displacement at the end portions of the vibrator small enough, there has been limitation in making the size of the vibrator small while securing sufficient amount of attenuation of the vibration displacement at the end portions of the vibrator.
Also, while the size of the vibrator could be made smaller by making the vibrator cross section bevel or convex in configuration as the amount of attenuation of the vibration displacement at the end portions of the vibrator becomes larger than that of a vibrator having a rectangular cross section, the vibrator suffered a problem of having a poor mechanical strength such as chipping or cracking of the end portions of the vibrator due to a small thickness of the end portions of the vibrator, thus making steady mounting difficult.
Furthermore, in the method of integrating the vibrator section and the support section at the end portions in the longitudinal direction of the vibrator and providing a slit on the end portions of the vibrator in the direction of width, there has been a problem of dispersion of the characteristic depending on the precision of working including chipping and surface roughness at the end portions in the direction of width of the vibrator.
FIG. 14 is a disassembled perspective view of an example of a conventional vibrator comprising a vibrator 29, a vibration-excitation electrode 30, a lead 31 for connection with an extemal-connection electrode 32, and a mounting board 33.